The invention relates to a device for mixing and dispensing bone, the device having a mixing cylinder, in which a mixing plunger is arranged, whereby the mixing plunger is axially movable by an actuation rod guided out in a sealed manner at a first cylinder end, and having a sealing plunger that is arranged in a region of the first cylinder end, is axially movable on the actuation rod and seals the mixing cylinder in a gas-tight manner. Moreover, the invention relates to a bone cement system having a device for mixing and dispensing bone cement as above, a reservoir for a binding agent, in particular a monomer, and a base, whereby the base stores the device and the reservoir.
Polymethylmethacrylate (PMMA) bone cements are based on the groundbreaking work of Sir Charnley (Charnley, J., “Anchorage of the femoral head prosthesis of the shaft of the femur,” J. Bone Joint Surg., 42: 28-30 (1960)). PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator (N,N-dimethyl-p-toluidine) dissolved therein. The powder component, which is also called bone cement powder, comprises one or more polymers that are made by polymerization, preferably suspension polymerization, based on methylmethacrylate and co-monomers, such as styrene, methylacrylate or similar monomers, a radio-opaquer, and the initiator, dibenzoylperoxide. When mixing the powder component with the monomer component, swelling of the polymers of the powder component in the methylmethacrylate leads to the formation of a dough that can be shaped plastically, corresponding to the actual bone cement. When mixing the powder component with the monomer component, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide while forming radicals. The radicals thus formed trigger the radical polymerization of the methylmethacrylate. Upon advancing polymerization of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies.
Polymethylmethacrylate bone cements can be mixed in suitable mixing beakers with spatulas by mixing the cement powder with the monomer liquid. This procedure is disadvantageous in that inclusions of air may be present in the cement dough thus formed and may later cause destabilization of the bone cement. For this reason, it is preferable to mix bone cement powder with the monomer liquid in vacuum mixing systems, since mixing in a vacuum removes inclusions of air from the cement dough all but completely and attains optimal cement quality (Breusch, S. J. et al., “Der Stand der Zementiertechnik in Deutschland [The State of Cementing Technology in Germany],” Z. Orthop., 137: 101-07 (1999)). Bone cements mixed in a vacuum have substantially lower porosity and thus show improved mechanical properties. A large number of vacuum cementing systems have been disclosed of which the following shall be named for exemplary purposes: U.S. Pat. Nos. 6,033,105; 5,624,184; 4,671,263; 4,973,168; 5,100,241; 5,586,821; and 5,344,232; International patent application Publication Nos. WO 99/67015 A1 and WO 94/26403 A1; European patent application publication Nos. EP 1 020 167 A2; EP 1 016 452 A2; EP 0 692 229 A1; and EP 1 005 901 A2; and German published patent application DE 36 40 279 A1. European patent application No. EP 1 920 738 A2 also describes a vacuum cementing system by which bone cements can be produced.
A further development of the aforementioned are cementing systems in which both the cement powder and the monomer liquid are already packed in separate compartments of the mixing systems and are mixed with each other in the cementing system only right before application of the cement (EP 0 692 229 A1). A significant problem of these systems is the sterilization of the entire system including the cement powder and the previously sterile-filtered monomer liquid. A particular problem in this context is the procedure of sterilization with ethylene oxide which is in common use for bone cements. Compared to sterilization with gamma radiation, this sterilization method is advantageous in that the polymers contained in the cement powder are not degraded and the properties of the cement remain unaffected by the ethylene oxide sterilization. A problem that is associated with ethylene oxide sterilization is that the gaseous agent first needs to penetrate into the cartridge and/or cement reservoir container and thus into the cement powder and then needs to diffuse out of the cartridge after sterilization is complete. Accordingly, it is obligatory for the gas exchange between the interior of the cartridge and/or the reservoir container and the surroundings to be as unimpeded as possible. In contrast, the ready-for use mixing system must be sealed sufficiently tightly for preparation-by-mixing of the cement in a vacuum to be feasible.
This conflict is resolved in mixing systems that are on the market in that a lid having a porous disc is screwed onto the cement cartridge and needs to be removed right before application of the cement. In place of the lid, a vacuum-tight cartridge head is screwed on, which contains a mixing device, a vacuum connection, and an opening for the dispensing tube to be attached later. The medical user therefore needs to open and then re-close the cementing system right before preparation-by-mixing of the cement. This may cause germs, etc., to enter into the previously disinfected bone cement powder.